The present invention relates to a silencer in magnetic field gradient coils in magnetic resonance imaging (MRI) apparatus, and more particularly to such silencer which reduces vibration noise generated by magnetic field gradient coils which generate magnetic field gradients to impart positional data to a magnetic resonance signal.
An MRI apparatus comprises a bed on which an object undergoing examination such as a human patient lies down, a measuring cylindrical bobbin having an internal space in which the bed is accommodated, a magnet which generates a stationary magnetic field to cause a nuclear magnetic resonance phenomenon in the object in the internal space, and a plurality of magnetic field gradient coils which generate various magnetic field gradients in the X, Y, Z axial directions. The magnetic field gradient coils are disposed along the outer periphery of the bobbin. In order to generate the magnetic field gradients, it is necessary to flow big pulse-like currents through the magnetic field gradient coils in which case the coils vibrate and generate a large percussion sound between the coils and the bobbin. This sound becomes noise which is very uncomfortable to an object undergoing examination or a patient.
Various attempts have been made to reduce the percussion noise produced between the bobbin and the coils at the generation of the magnetic field gradients. Main conventional means for reducing such noise is to use a damping material and/or a sound absorbing material in the bobbin.
A conventional noise reducer in an MRI apparatus is dedicated to reduction of only percussion noise occurring between the bobbin and the magnetic field gradient coils and traveling through the bobbin into the internal bobbin space or toward the patient. This applies, for example, to a silencer disclosed in JP-A-1-201247.
The applicants of the present invention have found that noise appearing in the external bobbin space is actually larger than that generated from the internal bobbin space when they examined the place where the noise was generated in the MRI apparatus.
A source of percussion noise is considered to be in a plane where the bobbin and the coils contact. Since the bobbin itself is a vibration transmitting member for sounds generated in the internal bobbin space, noise is easily reduced by attaching a buffering material to the bobbin. While a space is formed around the outer periphery of the bobbin, it is narrow, so that it is difficult to provide means for noise reducing purposes. In addition, provision of the space is necessary, so that no measures are conventionally taken for that purpose. Thus, even if noise reducing means is provided against noise appearing in the internal bobbin space, the noise cannot be reduced to a required enough level.